Medical CT equipment must undergo regular quality assessment (QA) testing to ensure the effective use thereof. Phantoms are commonly used tools for QA testing on CT equipment. The Catphan® 500 phantom (The Phantom Laboratory, Salem, N.Y., USA) is widely used in China, because it is recommended by China's existing verification regulation. The Catphan 500 phantom is mainly suitable for axial scanning of heads in size and function, and contains four testing modules therein, as shown in FIG. 1a and FIG. 1b. The Catphan 500 phantom is mainly composed of four testing modules that are used for testing image uniformity, low contrast resolution, high contrast resolution, and computed tomography (CT) number linearity, respectively. The high contrast resolution describes the capability of an imaging device to distinguish tightly close objects, and is also an important index for image performance in QA testing. In the Catphan 500 phantom, the high contrast resolution testing module CTP528 with an epoxy background is mainly composed of 21 aluminum line pair groups and 2 tungsten bead point sources (used for a point spread function (PSF) method) each having a diameter of 0.28 mm. The 21 line pair groups in the CTP528 module are arranged annularly. Each line pair group is expressed in the form of line pairs per centimeter (LP/cm), as shown in FIG. 1(c). One LP group consists of one or more pairs of black and white stripes having the same size, which represent (1-21) LP/cm, respectively. The PSF method has the advantages of simple structure and easy data processing, and the disadvantages of small data size, being seriously affected by noise, poor data reproducibility, and especially great difference between calculation results before and after image filtering.
A visual inspection method for high contrast resolution assessment presented in China's existing verification regulation is as follows: the window width (WW) of CT equipment is adjusted to be approximate to the minimum, and the window level (WL) of the CT equipment is adjusted to allow visual recognition of the smallest LP group. Such a visual inspection method is simple and fast, but has assessment results completely depending on the subjective consciousness of testers, with poor reproducibility and high uncertainty. In addition, in actual field tests, most of testers have insufficient experience, which further exacerbates the uncertainty of the test results.
In addition to the subjective assessment method, it is also presented in China's existing verification regulation that the built-in modulation transfer function (MTF) software of CT equipment under test is used for objective assessment. However, CT equipment from different manufactures almost all has their unique modulation transfer function (MTF) testing programs, which are difficult to standardize. The process of calling a program will certainly take up the running time of a machine, which causes a great trouble for QA testing. In spite of this, the MTF is always accepted as the gold standard for objective assessment on high contrast resolution in imaging systems; nevertheless, common image processing software hardly meets the use requirement of such a method on accurate positioning due to the annular structure of the CTP528 module. Therefore, the MTF method is poor in both operability and time efficiency.
Furthermore, in China, especially in large third-grade class-A hospitals, CT equipment may be used each day from dozens of times up to a hundred and even hundreds of times. Then, the performance of the CT equipment certainly declines gradually with the increase of exposures. According to the provisions of the verification regulation, the period of verification for CT equipment in use is one year; then, the high contrast resolution capability of medical CT equipment certainly changes, but this change is certainly accumulated over a long period. In the past, if a user of certain CT equipment wants to clearly control the change conditions of this CT equipment, the user needs to regularly draw an MTF curve by use of the MTF method and regard the value of the horizontal coordinate corresponding to the intersection point of the MTF curve and a threshold MTF10% as the high contrast resolution for this CT equipment. However, such a method is too difficult to operate, resulting in that a tester fails in testing at set time, and the CT equipment operators also cannot test the CT equipment.
In short, the subjective visual inspection method has excellent time efficiency, but is poor in accuracy and reproducibility, and the MTF method is good in both accuracy and reproducibility, but poor in operability and time efficiency.
In view of the defects of the above existing testing method for assessing the high contrast resolution, the inventor has made continuous research in order to create a novel objective method for assessing high contrast resolution of images based on Rayleigh criterion and make the method more practical.